1. Field of the Invention
The present invention relates to acoustic transducers for emitting or receiving acoustic waves, and to acoustic well logging apparatus incorporating such transducers.
2. Description of the Prior Art
Acoustic well logging apparatus for investigating the earth formations surrounding a borehole typically comprises an elongated support structure, or sonde, adapted to be moved longitudinally of the borehole, the support structure carrying at least one acoustic emitting transducer for emitting acoustic waves into the formations, and at least one acoustic receiving transducer, longitudinally spaced from the emitting transducer, for receiving the acoustic waves after they have passed through the formations. The emitting and receiving transducers currently used are generally similar to each other, and comprise a hollow cylindrical member which is open at both ends and which is normally mounted substantially coaxially with the support structure (and therefore substantially coaxially with the borehole). The cylindrical member is usually made from a piezoelectric or magnetostrictive material, and in order to use the transducer as an emitter, the cylindrical member is excited electrically or electromagnetically to cause it to vibrate resonantly, thereby generating the acoustic waves which are propagated through the formations. Similarly, when the transducer is used as a receiver, the acoustic waves incident on it cause it to vibrate resonantly, thereby generating an electric signal representative of the acoustic waves. The acoustic waves incident on the receiving transducer follow a path in which they are propagated generally outwardly from the emitting transducer, through the drilling mud normally present in the borehole, then refracted as they enter the formations to travel in the formations generally parallel to the borehole, and then refracted again to pass back through the drilling mud to the receiving transducer. In operation, the transit time of the acoustic waves, i.e. the time taken for them to travel between the emitting and receiving transducers, is measured, and various deductions about the properties of the formations are made from these measurements. In some acoustic well logging apparatus, a plurality of emitting and/or receiving transducers is used, in order to compensate for that part of the measured transit time due to the time taken by the acoustic waves to travel through the drilling mud and to reduce errors due to any eccentricity of the transducers in the borehole.
These currently used transducers suffer from the disadvantage that they emit their maximum acoustic energy in, or are most sensitive to acoustic energy arriving from, a plane perpendicular to the axis of the cylindrical member, i.e. a plane perpendicular to the borehole axis. This is due to the fact that, when the cylindrical member vibrates, it typically vibrates both radially and longitudinally, but by virtue of the relatively large area of the cylindrical outer surface of the member, only the radial vibrations are well coupled to the medium surrounding the member. But in acoustic well logging, the useful acoustic energy is typically that which is emitted and received at angles in the range of about 15.degree. to 75.degree. to the borehole axis, since it is this energy which is refracted at the drilling mud/formation interface to follow the path between the emitting and receiving transducers described hereinbefore.
To overcome this problem, it has already been proposed, in copending U.S. Pat. Application Ser. No. 256,075 filed on May 23, 1972 by J. C. Trouiller, now abandoned and continued as Ser. No. 986,076 filed on July 5, 1974, to mount the transducers with the axes of their cylindrical members perpendicular to the borehole axis, preferably closely adjacent the wall of the borehole. However, this proposal still does not increase the level of emitted energy, or the sensitivity to received energy, as much as is desired in the above range of angles.
It is known to mechanically couple one vibrating end of a transducer element with a mass which is intended to transmit or receive the compressional wave energy sent from, or received by, the transducer element. A device of this type is disclosed in U.S. Pat. No. 3,421,139 to J. A. Siebert, patented Jan. 7, 1969. However, as the mass has a flat emitting or receiving surface and this surface is of greater magnitude than the vibrating end of the transducer element to which it is coupled, this mass only increases the cross-section of the beam of acoustic energy that can be emitted or received, but the amount of emitted energy or the sensitivity to received energy in directions different from that of the vibrations of the transducer element is not increased.
It has also been proposed to mount the transducer with the axis of its cylindrical member substantially coaxial with the borehole and to mechanically couple the vibrating end of the transducer with a curved member that turns the path of the acoustic energy to direct it toward the wall of the borehole. A transducer of this sort is described in U.S. Pat. No. 3,138,219 to R. B Blizard, patented June 23, 1964. However, as this transducer has a directivity pattern which is directed to only one side of the borehole, it can investigate only a restricted part of the surrounding earth formations. As a consequence of this, an acoustic well logging device incorporating a transducer of this type is very sensitive to the mechanical problems associated with maintaining the required sonde position against the side of the borehole.
It is therefore a primary object of the present invention to provide an acoustic transducer suitable for use, either as an emitter or as a receiver of acoustic waves, in acoustic well logging apparatus, which transducer has an improved distribution of emitted energy, or has an increased sensitivity to received energy, in directions at angles of about 15.degree. to 75.degree. to the borehole axis.
It is another object of the present invention to provide an acoustic transducer having a directivity pattern located in the aforementioned range of angles, and substantially symmetrical with respect to the borehole axis.